During a semiconductor packaging process, a plurality of interconnected individual bare semiconductor chips or dice comprised in a wafer are often mounted onto an adhesive film stretched out on a wafer ring for singulation to separate the dice from one another. Mylar film is commonly used as the film with an adhesive surface for mounting the wafer. After singulation, the dice are individually picked up from the adhesive film and placed onto another die, a leadframe, laminate substrate or other carrier depending on the application. For automatically picking up the dice, a die ejection system is used to facilitate effective removal of the dice. In order to avoid the risk of die crack, partial delamination of the die from the adhesive film is advantageous before total removal of the die by a pick-head. The die ejection system thus has die ejector pins to lift a die from an opposite side of the adhesive film from its mounting side to partially delaminate the die from the adhesive film, and thereafter, a pick-head to remove the die totally from the adhesive surface.
Traditionally, die bonding machines use die ejection systems based on some kind of transmission/linkage mechanism used to convert rotational motion of a rotary motor into translational motion used for facilitating ejection of a die. For example, U.S. Pat. No. 5,755,373 for a “Die Push-Up Device” discloses a mechanism including a push-up needle raised and lowered by a cam that is actuated by a rotary motor.
FIG. 1 is a simplified diagram showing various parts of a conventional ejection system in greater detail. The ejection pins 1 are held by a collet 1a, which is mounted at the end of a shaft 2. A roller 15 is affixed to the lower end of the shaft. A rotary motor 20 is used to drive a high precision cam 16, which actuates the shaft 2 through the roller 15 in order to reduce friction. Typically a step-down transmission consisting of a timing belt 18 and pulleys 17, 19 is used to drive the cam 16. Any other suitable transmission mechanism may be used.
FIG. 2 shows the typical operation of ejector pins used to induce partial delamination of dice mounted on an adhesive Mylar sheet. As shown in FIG. 2a, the ejector pins 1 are normally positioned just under a plurality of dice 22 that have been mounted on an adhesive surface of the Mylar sheet 21. This is to allow a wafer table holding the Mylar sheet 21 to execute free indexing motion in the horizontal plane without any obstruction from the pins 1. After the wafer table executes an indexing motion so as to position a die at the pick position on the ejector platform 23, the ejector pins 1 move up as shown in FIG. 2b and at the same time, vacuum force 24 in the direction indicated by dotted arrows is applied from below the Mylar sheet 21. The ejector pins 1 contact the Mylar sheet 21 and lift the die 22 along with the sheet. However, strong vacuum applied from below serves to keep some areas of the Mylar sheet 21 in close contact with the top surface of the ejector platform 23. A rubber seal 25 is provided to concentrate the vacuum around the area of the pick-up position.
In relation to the conventional die ejector system described above, the presence of several components between the drive motor 20 and the end-effector i.e. ejector pin array 1 introduces substantial compliance (reciprocal of stiffness), friction, backlash and hysteresis problems in the system. This reduces the control bandwidth thus severely limits the performance of the ejector system. Secondly, it is difficult to control the impact of the pins on the die during ejection. This could lead to cracking of the die, more so as ever-thinner dies are being introduced into use in the semiconductor industry.
Therefore, it would be desirable to implement a directly driven ejector mechanism for better control of the motion of the ejector pins 1. Moreover, it would also be useful to have a flexure bearing design associated with the directly driven ejector mechanism to improve accuracy and repeatability of the motion.